The invention refers to a method of controlling the pressure supply of a hydraulic piston-cylinder unit connected to a pressure source, which performs a working operation comprising alternating working strokes and return strokes, as well as to a hydraulic drive means for controlling a piston-cylinder unit.
Piston-cylinder units are often used to perform a working operation with repeated working strokes and return strokes. For example, such an application is found in power wrenches where a ratchet wheel is rotated by a piston-cylinder unit through a detent that turns the ratchet wheel further on by one angular segment each time. Generally, there is a problem to detect when the working operation is terminated and the piston-cylinder unit should be retracted or left in its position. Detecting the end of a working operation, where a defined force has been exerted on a load, is difficult in practice. In general, this detection is made by an operator who determines whether the piston-cylinder unit is still moving. The stopping of the movement may be due, on the one hand, to the fact that the requested force has been exerted on the load, but also to the fact that the piston of the piston-cylinder unit has reached its end position. In a hydraulic power wrench, the user has to watch the turning of the screw. A user does not see, whether the screw is fixed or the piston-cylinder unit has reached the piston end position. Upon standstill of the piston-cylinder unit, the user has to decide whether turning the screw should be stopped because the desired torque has been reached or whether another working stroke should be performed. Thus, the operation of such a hydraulic tool is subject to the judgment of the user. This is disadvantageous. The working operation is not sufficiently secure, i.e. reaching the desired torque is not guaranteed. Neither is the working operation performed in minimum time, but it may happen that after the end of a working operation another working stroke is attempted. Further, the user must continuously be alert, resulting in high personnel costs.